The present invention relates to a positioning device, and more specifically, it relates to a positioning device capable of precise positioning and force control along the Z-axis (the vertical axis) direction, and a processing machine using the same.
One known application of a positioning device having a positioning function with respect to the vertical direction is a chip mounter. The chip mounter will be described below with reference to FIG. 1.
In FIG. 1, the chip mounter includes a Z-axis feeder 100, and a holder support unit 200 driven along an up-and-down direction by the Z-axis feeder 100. In the Z-axis feeder 100, a servomotor 102 is provided in a device frame 101, and it drives a ball screw mechanism 103 extending along the vertical direction. The ball screw mechanism 103 has a slider 104. The slider 104 is guided by a guide rail 105 provided in the device frame 102.
The holder support unit 200 is a movable section in the ball screw mechanism 103, and is installed in a position opposite to the slider 104 via a holder bracket 201. The holder support unit 200 has an air cylinder 202. In the air cylinder 20, there is provided a tool holder 203 via a hydrostatic bearing 204 so as to be movable along the up-and-down direction. The hydrostatic bearing 204 is for supporting a lower portion of the tool holder 203 in a non-contact state. For this purpose, the hydrostatic bearing 204 receives compressed air supplied from a hole 205 provided in the air cylinder 202, and uniformly disperses the compressed air through a porous member, thereby blowing it onto the outer surface of the tool holder 203.
The up-and-down movement of the tool holder 203 is position-controlled by the differential pressure between the pressure of compressed air supplied from a pressing port 206 opened to the air cylinder 202 and the pressure of compressed air supplied from a balance pressure port 207 opened to the air cylinder 202. At the lower end of the tool holder 203, there is provided a tool 209 for holding a chip 300.
A substrate 401 on which the chip 300 is to be mounted is placed on a substrate holding stage 400. Here, the air cylinder 202 has a position detector 210 for detecting a moving height thereof. The detection signal of the position detector 210 is used for feedback control of the height position.
The above-described chip mounter is disclosed in Japanese Unexamined Patent Publication (JP-A) No. 2000-353725.
In order to allow precise positioning, this chip mounter requires two drive mechanisms: the Z-axis feeder 100 using the ball screw mechanism 103 and the holder support unit 200 using the air cylinder 202.
One possible alternative drive source to the combination of the above-described two drive mechanisms is a linear motor. However, with regard to the chip mounter, its movement is so vigorous that the electromagnet in the linear motor produces heat. This can unfavorably have a detrimental effect on a chip. In addition, when performing positioning control in the Z-axis direction like the chip mounter, it is necessary to perform not only positioning control but also force control (load control) of the movable section. However, in the linear motor, it is difficult to realize force control by current control.
Meanwhile, another patent application assigned to the same assignee as this application proposes the following hydraulic actuator as an alternative drive source to the drive mechanism using the ball screw mechanism or that using the linear motor as described above (see Japanese Unexamined Patent Publication (JP-A) No. 2002-295404).
FIG. 2 is a constructional view of this hydraulic actuator. As shown in FIG. 2, the hydraulic actuator includes a guide shaft 414 and a slider 413 movable therealong. Formed between the guide shaft 414 and the slider 413 is a cylinder chamber. A pressure receiving plate 417 is provided in the slider 413 for dividing the cylinder chamber into two pressure chambers 416A and 416B with respect to the moving direction. By allowing compressed air to enter and exit the two-divided pressure chambers 416A and 416 via servo valves 422A and 422B, respectively, the slider 413 is driven by the differential pressure between the two pressure chambers 416A and 416B.
The hydraulic actuator further includes a position sensor 415 for detecting the position of the slider 413, two servo amplifiers 421A and 421B for controlling the two servo valves 422A and 422B, respectively, and a control computing unit 420 for receiving a position detection signal from the position sensor 415 to output respective position command values to the two servo amplifiers 421A and 412B. Reference numeral 410 denotes a compressed air supply source.
The control computing unit 420 executes the step of calculating a velocity of the slider 413 by differentiating the slider position indicated by the position detection signal, and calculating an acceleration thereof by differentiating the calculated velocity. The control computing unit 420 also executes the step of calculating respective position command values to be outputted to the two servo amplifiers 421A and 421B by using a slider target position, a slider position, a slider velocity, and a slider acceleration. The control computing unit 420 further executes the step of performing computation to compensate for the respective pressure changes of the pressure chambers 16A and 16B due to position changes of the pressure receiving plate 417 in the cylinder chamber, with respect to the respective calculated position command values, and outputting the respective compensated position command values to the two servo amplifiers 421A and 421B, respectively.
In general, a pneumatic actuator using air as a fluid has an advantage that it can provide a high velocity and a high thrust, and that it is low in heating action. However, although the arrangement as described above is suitable for a drive source in the horizontal direction, it is unsuitable for a drive source in the vertical direction, namely, the Z-axis direction. In addition, this type of arrangement requires two expensive servo valves.